Process to prepare base oil from a fisher-tropsch synthesis product

ABSTRACT

A process to prepare base oils from a Fischer-Tropsch synthesis product by (a) separating the Fischer-Tropsch synthesis product into a fraction (i) boiling in the middle distillate range and below, a heavy ends fraction (iii) and an intermediate base oil precursor fraction (ii) boiling between fraction (i) and fraction (iii), (b) subjecting the base oil precursor fraction (ii) to a catalytic hydroisomerisation and catalytic dewaxing process to yield one or more base oil grades, (c) subjecting the heavy ends fraction (iii) to a conversion step to yield a fraction (iv) boiling below the heavy ends fraction (iii) and (d) subjecting the high boiling fraction (v) of fraction (iv) to a catalytic hydroisomerisation and catalytic dewaxing process to yield one or more base oil grades.

The present invention is directed to a process to prepare base oils orthe intermediate waxy raffinate product in a high yield from aFischer-Tropsch synthesis product.

Such processes are known from WO-A-9941332, U.S. Pat. No. 6,080,301,EP-A-0668342, U.S. Pat. No. 6,179,994 or WO-A-02070629. These processesall comprise some kind of hydroisomerisation of the Fischer-Tropschsynthesis product followed by a dewaxing step of the higher boilingfraction obtained in said hydroisomerisation.

WO-A-02070629, for example, describes a process wherein the C5 plusfraction of a Fischer-Tropsch synthesis product is first subjected to ahydrocracking/hydroisomerisating step in the presence of a catalystconsisting of platinum on an amorphous silica-alumina carrier. Theeffluent of this conversion step is separated into middle distillateproducts and a base oil precursor fraction and a higher boilingfraction. The base oil precursor fraction is catalytically dewaxed inthe presence of a platinum-ZSM-5 based catalyst and the heavy fractionis recycled to the hydrocracking/hydroisomerisating step.

Although such a process will yield excellent quality base oils there isroom for improvement. Especially the yield of base oils relative to theFischer-Tropsch synthesis product may be improved. Especially for baseoils having a kinematic viscosity at 100° C. of between 2 and 8 cSt animproved yield would be welcome.

The present invention aims at providing such a process.

The following process achieves this object. Process to prepare base oilsfrom a Fischer-Tropsch synthesis product by

(a) separating the Fischer-Tropsch synthesis product into a fraction (i)boiling in the middle distillate range and below, a heavy ends fraction(iii) and an intermediate base oil precursor fraction (ii) boilingbetween fraction (i) and fraction (iii),

(b) subjecting the base oil precursor fraction (ii) to a catalytichydroisomerisation and catalytic dewaxing process to yield one or morebase oil grades,

(c) subjecting the heavy ends fraction (iii) to a conversion step toyield a fraction (iv) boiling below the heavy ends fraction (iii) and

(d) subjecting the high boiling fraction (v) of fraction (iv) to acatalytic hydroisomerisation and catalytic dewaxing process to yield oneor more base oil grades.

Applicants have found that by directly subjecting the fraction of theintermediate fraction (ii) of the Fischer-Tropsch synthesis product andthe high boiling fraction (v) as obtained in step (c) to a selectiveisomerisation and dewaxing step a higher yield to base oils relative tothe Fischer-Tropsch synthesis product can be obtained.

Without intending to be bound by the following theory it is believedthat the high yield to base oils is achieved in that the fractionboiling in the base oil range, i.e. fractions (ii) and (v) are directlycontacted with he catalytic isomerisation and dewaxing catalysts. In theprior art process of WO-A-02070629 the corresponding fraction of theFischer-Tropsch synthesis product was first contacted with a catalystwhich would convert a large part to middle distillate products and lowerboiling products. By using this different line-up the conversion ofpotential base oil molecules in the Fischer-Tropsch synthesis product tomiddle distillate molecules is minimized. Furthermore in the process ofWO-A-02070629 the heavy fraction as obtained in thehydrocracking/hydroisomerisating step is recycled to said step. Thisresults in that more potential base oil molecules are converted tomiddle distillate molecules.

The Fischer-Tropsch synthesis product can be obtained by well-knownprocesses, for example the so-called commercial Sasol process, the ShellMiddle Distillate Process or by the non-commercial Exxon process. Theseand other processes are for example described in more detail inEP-A-776959, EP-A-668342, U.S. Pat. No. 4,943,672, U.S. Pat. No.5,059,299, WO-A-9934917 and WO-A-9920720. Typically theseFischer-Tropsch synthesis products will comprise hydrocarbons having 1to 100 and even more than 100 carbon atoms. The hydrocarbon product willcomprise iso-paraffins, n-paraffins, oxygenated products and unsaturatedproducts. The feed to step (a) or any fractions obtained in step (a) maybe hydrogenated in order to remove any oxygenates or unsaturatedproducts. The process of the present invention is especiallyadvantageous when a substantial part, preferably more than 10 wt %, morepreferably more than 30 wt % and even more preferably more than 50 wt %of the Fischer-Tropsch synthesis product boils above 550° C. An exampleof a suitable process which may prepare such a heavy Fischer-Tropschsynthesis product is described in WO-A-9934917 and in AU-A-698392.

In step (a) the Fischer-Tropsch synthesis product is separated into afraction (i) boiling in the middle distillate range and below, a heavyends fraction (iii) preferably, having an initial boiling point between500 and 600° C. and an intermediate base oil precursor fraction (ii)boiling between fraction (i) and fraction (iii). Suitably theFischer-Tropsch synthesis product is first fractionated at atmosphericpressure or higher to obtain fraction (i) boiling in the middledistillate range and below. Fractionation may be performed by flashingor distillation. The middle distillate range is sometimes defined as thefraction boiling predominately, i.e. for more than 90 wt %, between 200and 350° C. and it comprises the gas oil and kerosene fractions, whichcan be isolated from the Fischer-Tropsch synthesis product. The residueor bottom product of the atmospheric fractionation is further separatedat near vacuum pressure to the heavy ends fraction (iii) having aninitial boiling point between 500 and 600° C. and the intermediate baseoil precursor fraction (ii). More preferably the T10 wt % recovery pointof the heavy ends fraction (iii) is between 500 and 600° C.

In step (b) the base oil precursor fraction (ii) is subjected to acatalytic hydroisomerisation and catalytic dewaxing process to yield oneor more base oil grades. These catalytic processes are defined accordingto this invention as processes, which are selective for reducing thepour point of this fraction while minimising the conversion of moleculesboiling above 370° C. to molecules boiling below 370° C. It should benoted that when lower temperature pour points are desired for the baseoil, more molecules will, even by the more selective isomerisation anddewaxing processes, be converted to fractions boiling below 370° C.Selective isomerisation and dewaxing processes are preferably processeswherein less than 40 wt %, more preferably less than 30 wt %, of thefeed to step (b) is converted to a fraction boiling below 370° C. whenpreparing a base oil having a kinematic viscosity at 100° C. of 5 cStand having a pour point of −27° C. and a Noack volatility of 10 wt %.Examples of processes having the above described selectivity to baseoils are well known and will be described below.

In step (b) the catalytic hydroisomerisation and catalytic dewaxing maybe performed by one catalyst or by separate dedicated isomerisation anddewaxing catalysts.

Examples of possible isomerisation catalysts comprise one or more GroupVIII metal, for example nickel, cobalt platinum or palladium on arefractory oxide carrier. Examples of specific catalysts are Pt onsilica-alumina carrier (ASA), Pd on ASA, PtNi on ASA, PtCo on ASA, PtPdon ASA, CoMoCu on ASA, NiMoCu on ASA, NiW on ASA, NiWF on alumina, PtFon ASA, NiMoF on alumina. As a separate dewaxing step use can be made instep (b) of the well known catalytic dewaxing processes whereincatalysts are used comprising medium pore size molecular sieves and ahydrogenation component, preferably a noble metal such as platinum orpalladium. Examples of such processes are those based on SAPO-11 asdescribed in for example EP-A-458895, ZSM-5 as for example described inEP-B-832171, ZSM-23 as described in for example EP-A-540590 andEP-A-092376, ZSM-22 as for example described in U.S. Pat. No. 4,574,043,mordenite as for example described in U.S. Pat. No. 6,179,994 andferrierite as for example described in EP-A-1029029.

Such separate isomerisation processes in combination with a catalyticdewaxing process is for example described in EP-A-776959.

If step (b) is carried out using a single catalyst process, catalyst maybe used based on for example catalysts comprising platinum-zeolite-beta,as described in for example U.S. Pat. No. 5,885,438, or ZSM-23 or ZSM-22based catalysts as for example described in EP-A-536325. Preferably useis made of a process which makes use of a ZSM-12 based catalyst as forexample described in WO-A-0107538.

Advantageously an isomerisation step using a catalyst based onzeolite-beta is combined with a selective catalytic dewaxing stepwherein use can be made of the dewaxing catalysts described above.Examples of the use of platinum-zeolite-beta catalysed step followed bya more selective dewaxing step using a platinum-ZSM-23 catalysed step isfor example in U.S. Pat. No. 4,919,788 and EP-A-1029029. The ZSM-23,ZSM-22 and ZSM-12 catalysts may also be used in a cascade dewaxingoperation wherein the final dewaxing is performed making use of a morerestricted pore size zeolite like for example ZSM-5, ZSM-11 orferrierite as for example described in U.S. Pat. No. 4,599,162.

A most preferred process for step (b) comprises contacting the fraction(ii) with a catalyst comprising ZSM-12, platinum and a binder. It hasbeen found that this process achieves a high selectivity to base oils.Another advantage is less gaseous by-products and more gas oilby-products are made. Preferably the binder is a low acidity bindercomprising essentially no alumina. Preferably the binder is silica. Morepreferably the zeolite is dealuminated and more preferably adealumination treatment is chosen which claims to selectivelydealuminate the surface of the ZSM-12 crystallites. Such catalysts andthe process conditions for performing this process are described in moredetail in WO-A-0107538.

After performing a dewaxing step (b) the desired base oil(s) arepreferably isolated from the dewaxed effluent in a base oil recoverystep (e). In this step (e) lower boiling compounds formed duringcatalytic dewaxing are removed, preferably by means of distillation,optionally in combination with an initial flashing step. By choosing asuitable distillation cut as feed to step (b) in step (a) it is possibleto obtain a single desired base oil directly after a catalytic dewaxingstep (b) without having to remove any higher boiling compounds from theend base oil grade. Examples of very suitable grades are base oilshaving a kinematic viscosity at 100° C. of between 3.5 and 6 cSt.

It has also been found possible to make more than one viscosity gradebase oil with the process according to the invention. By obtaining abase oil precursor fraction (ii) in step (a) having a more broad boilingrange more base oil grades may advantageously be obtained in step (e).Preferably the difference between the T10 wt % recovery point and the 90wt % recovery point in the boiling curve is larger than 100° C. In thismode the effluent of step (b) is separated into various distillatefractions comprising two or more base oil grades. In order to meet thedesired viscosity grades and volatility requirements of the various baseoil grades preferably off-spec fractions boiling between, above and/orbelow the desired base oil grades are also obtained as separatefractions. These fractions and any fractions boiling in the gas oilrange or below may advantageously be recycled to step (a). Alternativelyfractions obtained boiling in the gas oil range or below may suitably beused as a separate blending component to prepare a gas oil fuelcomposition.

The separation into the various fractions in step (e) may suitably beperformed in a vacuum distillation column provided with side strippersto separate the fraction from said column. In this mode it is foundpossible to obtain for example a 2-3 cSt product, a 4-6 cSt product anda 7-10 cSt product simultaneously from a single base oil precursorfraction (ii). The viscositie values are the kinematic viscosity at 100°C.

In step (c) the heavy ends fraction (iii) is subjected to a conversionstep to yield a fraction (iv) boiling below the heavy ends fraction(iii). Step (c) may be performed by any conversion process capable ofconverting the heavy Fischer-Tropsch wax to lower boiling hydrocarboncompounds. If the conversion product of step (c) is to contain a highcontent of olefinic compounds preferably a conversion process is appliedwhich operates in the absence of added hydrogen. Examples of suitableprocesses which operate in the absence of added hydrogen are the wellknown thermal cracking process as for example described in U.S. Pat. No.6,703,535 and the catalytic cracking process as for example described inU.S. Pat. No. 4,684,759. If on the other hand the conversion product ofstep (c) is to contain almost no olefins preferably a process is appliedwhich is performed in the presence of added hydrogen. An example of asuitable process is the well known hydroisomerisation/hydrocrackingprocess.

Preferably a hydrocracking/hydroisomerisation reaction takes place instep (c). Step (c) is preferably performed in the presence of hydrogenand a catalyst, which catalyst can be chosen from those known to oneskilled in the art as being suitable for this reaction. Examples of suchcatalysts are the isomerisation catalysts as described above whendiscussing step (b). Catalysts for use in step (c) typically areamorphous catalysts comprising an acidic functionality and ahydrogenation/dehydrogenation functionality. Preferred acidicfunctionality's are refractory metal oxide carriers. Suitable carriermaterials include silica, alumina, silica-alumina, zirconia, titania andmixtures thereof. Preferred carrier materials for inclusion in thecatalyst for use in the process of this invention are silica, aluminaand silica-alumina. A particularly preferred catalyst comprisesplatinum, platinum or platinum and palladium supported on asilica-alumina carrier. If desired, the acidity of the catalyst carriermay be enhanced by applying a halogen moiety, in particular fluorine orchlorine, or a phosphorous moiety to the carrier. Examples of suitablehydrocracking/hydroisomerisation processes and suitable catalysts aredescribed in WO-A-200014179, EP-A-532118 and the earlier referred toEP-A-776959.

Preferred hydrogenation/dehydrogenation functionality's are Group VIIInon-noble metals, for example nickel and more preferably Group VIIInoble metals, for example palladium and most preferably platinum orplatinum and palladium. The catalyst may comprise thehydrogenation/dehydrogenation active component in an amount of from0.005 to 5 parts by weight, preferably from 0.02 to 2 parts by weight,per 100 parts' by weight of carrier material. A particularly preferredcatalyst for use in the hydroconversion stage comprises platinum in anamount in the range of from 0.05 to 2 parts by weight, more preferablyfrom 0.1 to 1 parts by weight, per 100 parts by weight of carriermaterial. The catalyst may also comprise a binder to enhance thestrength of the catalyst. The binder can be non-acidic. Examples arealumina, silica, clays and other binders known to one skilled in theart.

In step (c) the feed is contacted with hydrogen in the presence of thecatalyst at elevated temperature and pressure. The temperaturestypically will be in the range of from 175 to 380° C., preferably higherthan 250° C. and more preferably from 300 to 370° C. The pressure willtypically be in the range of from 10 to 250 bar and preferably between20 and 80 bar. Hydrogen may be supplied at a gas hourly space velocityof from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. Thehydrocarbon feed may be provided at a weight hourly space velocity offrom 0.1 to 5 kg/l/hr, preferably higher than 0.5 kg/l/hr and morepreferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbonfeed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500Nl/kg.

The conversion in step (c) as defined as the weight percentage of thefeed boiling above 370° C. which reacts per pass to a fraction boilingbelow 370° C. is preferably at least 20 wt %, more preferably at least25 wt %, preferably not more than 80 wt %, more preferably not more than70 wt % and even more preferably not more than 65 wt %.

Step (c) may also be performed making use of a catalyst comprising amolecular sieve and a metal hydrogenation component. Examples ofsuitable molecular sieves are SAPO-11, ZSM-22 or ZSM-23. Preferably themolecular sieve has a pore structure of the 12-oxygen ring type.Suitable molecular sieves having a 12-member ring structure for use inthe present invention are zeolite beta and ZSM-12. Suitablehydrogenation metals are preferably of Group VIII of the periodic tableof elements. More preferably the hydrogenation component is nickel,cobalt and even more preferably platinum or palladium. Examples of theabove catalysts are described in more detail above at step (b). Anadvantage of using a molecular sieve based type catalyst in step (c) isthat an additional dewaxing of the resultant base oil may be omitted. Inessence step (c) and step (d) are thus combined in one step. It has beenfound that very high viscosity base oil grades, preferably having akinematic viscosity at 100° C. of above 15 cSt, may be prepared in thismanner. The maximum viscosity will depend on the heaviness of theFischer-Tropsch synthesis product used as feed in step (a).

Preferably the effluent of the above combined steps (c) and (d) isprovided to the same above described base oil work up section (step(e)). This is advantageous because the isolation of all base oil grades,including the heavier grade, may then be performed in the samedistillation column(s).

In the event step (c) is performed in the presence of a molecular sievebased catalyst the following process conditions are generally applied.The temperatures typically will be in the range of from 175 to 380° C.,preferably higher than 200° C. and more preferably from 220 to 330° C.The required temperature will partly depend on the acidity of themolecular sieve which may vary per type of molecular sieve and thedegree of dealumination. A skilled person can easily find the optimaltemperature conditions. The pressure will typically be in the range offrom 10 to 250 bar and preferably between 20 and 80 bar. Hydrogen may besupplied at a gas hourly space velocity of from 100 to 10000 Nl/l/hr,preferably from 500 to 5000 Nl/l/hr. The hydrocarbon feed may beprovided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr,preferably higher than 0.5 kg/l/hr and more preferably lower than 2kg/l/hr. The ratio of hydrogen to hydrocarbon feed may range from 100 to5000 Nl/kg and is preferably from 250 to 2500 Nl/kg.

If step (c) is performed by means of a thermal cracking of catalyticcracking process the effluent will contain relatively high contents ofolefinic compounds boiling below 370° C. and especially in the fractionboiling in the gasoline range of between 25 and 215° C. These fractionsare obtained as the remaining lower boiling fraction (vi) when isolatingby distillation the high boiling fraction (v) from fraction (iv). Theolefin content in these fractions may range up to and above 50 wt %.These olefins include propylene which can be used as a chemicalfeedstock, C₄-olefins which can be used as alkylation feedstock and C₅and higher olefins which can be used as feed to an oligomerisation stepto increase the yield to higher boiling products such as gas oil andbase oils. These olefins are advantageous if one intends to prepare agasoline blending fraction or as feedstock to an alkylation process step(g) to prepare gasoline blending compounds from C₄-olefins andC₄-paraffins present in such fractions or as feedstock for an additionaloligomerisation step (f) to prepare compounds boiling in the gas oil andbase oil boiling range.

The suitable thermal cracking step employed in step (c) is intended tocrack the paraffin molecules into lower molecular weight olefins. Theprocess may be performed in the liquid or vapor phase. Examples ofliquid phase process configurations are the batch pyrolysis reactorssuch as employed in delayed coking or in cyclic batch operations. Theprocess may also be carried out in the gas phase wherein a continuousflow-through operation is preferred. In such a process the feed is firstpreheated to a temperature sufficient to vaporize most or all of thefeed after which the vapor is passed through a tube or tubes. Adesirable option is to bleed any remaining nonvaporized hydrocarbonsprior to entering the tubes in the cracking furnace. Preferably, thethermal cracking is conducted in the presence of steam, which serves asa heat source and also helps suppress coking in the reactor. Details ofa typical steam thermal cracking process may be found in U.S. Pat. No.4,042,488.

In performing the gas phase thermal cracking operation it is preferablethat the feed be maintained in the vapor phase during the crackingoperation to maximize the production of olefins. In the pyrolysis zone,the cracking conditions should be sufficient to provide a crackingconversion of greater than 30% by weight of the paraffins present.Preferably, the cracking conversion will be at least 50% by weight andmost preferably at least 70% by weight. The optimal temperature andother conditions in the pyrolysis zone for the cracking operation willvary somewhat depending on the feed. In general, the temperature must behigh enough to maintain the feed in the vapor phase but not so high thatthe feed is overcracked. The temperature in the pyrolysis zone normallywill be maintained at a temperature of between 500° C. and 900° C. Theoptimal temperature range for the pyrolysis zone in order to maximizethe production of olefins from the Fischer-Tropsch wax will depend uponthe endpoint of the feed. In general, the higher the carbon number, thehigher the temperature required to achieve maximum conversion. Thepyrolysis zone usually will employ pressures maintained between about 0atmospheres and about 5 atmospheres, with pressures in the range of fromabout 0 to about 3 generally being preferred. Although the optimalresidence time of the wax fraction in the reactor will vary depending onthe temperature and pressure in the pyrolysis zone, typical residencetimes are generally in the range of from about 0.1 seconds to about 500seconds, with the preferred range being between about 0.1 and 5 seconds.

In the event step (c) is performed using a catalytic cracking process,of which the fluid catalytic cracking (FCC) process is an example, thefollowing conditions are preferred. Preferably the feed will becontacted with a catalyst at a temperature between 450 and 650° C. Morepreferably the temperature is above 475° C. The temperature ofpreferably below 600° C. to avoid excessive overcracking to gaseouscompounds. The process may be performed in various types of reactors.Because the coke make is relatively small as compared to a FCC processoperating on a petroleum derived feed it is possible to conduct theprocess in a fixed bed reactor. In order to be able to regenerate thecatalyst more simple preference is nevertheless given to either afluidized bed reactor or a riser reactor. If the process is performed ina riser reactor the preferred contact time is between 1 and 10 secondsand more preferred between 2 and 7 seconds.

The catalyst to oil ratio is preferably between 2 and 20 kg/kg. It hasbeen found that good results may be obtained at low catalyst to oilratio's of below 15 and even below 10 kg/kg.

The catalyst system used in the catalytic cracking process in step (c)will at least comprise of a catalyst comprising of a matrix and a largepore molecular sieve. Examples of suitable large pore molecular sievesare of the faujasite (FAU) type as for example Zeolite Y, Ultra StableZeolite Y and Zeolite X. The matrix is preferably an acidic matrix.Examples of suitable catalysts are the commercially available FCCcatalysts. The catalyst system process may advantageously also compriseof a medium pore size molecular sieve such to also obtain a high yieldof propylene next to the gasoline fraction. Preferred medium pore sizemolecular sieves are zeolite beta, Erionite, Ferrierite, ZSM-5, ZSM-11,ZSM-12, ZSM-22, ZSM-23 or ZSM-57. The weight fraction of medium porecrystals on the total of molecular sieves present in this process ispreferably between 2 and 20 wt %.

In step (d) the high boiling fraction (v) of fraction (iv) is subjectedto a catalytic hydroisomerisation and catalytic dewaxing process toyield one or more base oil grades. The high boiling fraction (v) in theeffluent of step (c) preferably has a initial boiling point of between340 and 400° C. More preferably the 10 wt % recovery point is between340 and 400° C. The final boiling point of said fraction (v) ispreferably between 500 and 600° C. More preferably the 90 wt % recoverypoint is between 500 and 600° C. Step (d) may be performed as describedabove for step (b). Separations are preferably performed by means ofdistillation. Preferably the base oils are isolated from the effluent ofstep (d) in the same base oil work-up section (step (e)) as describedabove.

Preferably the effluent of step (c) is provided to step (a). This isadvantageous because it reduces the number of distillation columns. Instep (a) a mixture of fresh Fischer-Tropsch synthesis product and step(c) effluent will be separated simultaneously into again a fraction (i)boiling in the middle distillate range and below, a heavy ends fraction(iii) and an intermediate base oil precursor fraction (ii) boilingbetween fraction (i) and fraction (iii). In this embodiment step (b) and(d) are performed in the same reactor, which is also advantageous forobvious reasons.

The Fischer-Tropsch synthesis product may contain olefins and oxygenateswhich may be detrimental for the hydroconversion catalysts used in step(b), (c) and (d).

These compounds may be removed by means of hydrogenation of theFischer-Tropsch synthesis product prior to performing step (a) orhydrogenation of the feeds to the separate steps (b), (c) and/or (d).The latter is advantageous because some of the oxygenates present in theFischer-Tropsch synthesis product will end up in the middle distillatefraction (i) and could serve as lubricity enhancers in the resulting gasoil or kerosene fractions. The advantages of the presence of suchcompounds are for example described in EP-A-101553.

Possible hydrogenation processes are for example described inEP-B-668342. The mildness of the hydrotreating step is preferablyexpressed in that the degree of conversion in this step is less than 20wt % and more preferably less than 10 wt %. The conversion is heredefined as the weight percentage of the feed boiling above 370° C.,which reacts to a fraction boiling below 370° C. Examples of possiblehydrogenation processes involve the use of nickel containing catalysts,for example nickel on alumina, nickel on silica-alumina nickel onKieselguhr, copper nickel on alumina, cobalt on silica-alumina orplatinum nickel on alumina. The hydrogenation conditions are typicalconditions for these type of processes, well known to the skilledperson.

The above referred to oligomerisation step (f) is intended to maximizethe yield of base oils by oligomerizing the olefins present in thefraction boiling below 370° C. and especially boiling in the gasolinerange as obtained in step (c). In a preferred embodiment the olefiniceffluent of step (c) is provided to step (a) to obtain an olefinicfraction (i) which can be directly used in step (f) or may be furtherfractionated before being used in step (f). In such a scheme the feed tostep (f) will also comprise the lower boiling compounds as present inthe Fischer-Tropsch synthesis product.

Because such a product may also contain olefins oligomerization of theseolefins will then also take place in step (f). During oligomerizationthe lighter olefins are converted into heavier products. The moleculesboiling in the base oil range as obtained in step (f) will haveexcellent viscosity index making them suitable to be blended with thebase oil products as obtained in steps (b) and (d). In the event thepour point is too high, the oligomerization product may be send to acatalytic dewaxing step. In a preferred embodiment this dewaxing can beperformed by co-feeding this fraction with the feed of step (b) oralternatively of step (d) to their respective dewaxing processes. Evenmore preferred is to send the oligomerization product to step (a) inorder to simplify the number of distillation steps before performing thedewaxing in step (b). In order to avoid a build up of paraffins boilingin the same boiling range as the olefins which are converted a bleedstream is preferably present. In a preferred embodiment step (f) isperformed in a catalytic distillation in which simultaneously theolefins are converted to base oil molecules, which due to their higherboiling point are recovered at the bottom of the distillation column.Unreacted olefins are obtained at or near the top of the column and canbe recycled to the catalytic distillation column.

The oligomerization of olefins has been well reported in the literature,and a number of processes are available. See, for example, U.S. Pat. No.670,693, US-A 20040029984, U.S. Pat. No. 6,703,535, U.S. Pat. No.4,417,088, U.S. Pat. No. 4,434,308, U.S. Pat. No. 4,827,064, U.S. Pat.No. 4,827,073 and U.S. Pat. No. 4,990,709. Various types of reactorconfigurations may be employed, with the fixed catalyst bed reactorbeing used commercially. More recently, performing the oligomerizationin an ionic liquids media has been proposed, since the contact betweenthe catalyst and the reactants is efficient and the separation of thecatalyst from the oligomerization products is facilitated. But also thecatalytic distillation process may be used advantageously.

Preferably, the oligomerized product will have an average molecularweight at least 10 percent higher than the initial feedstock, morepreferably at least 20 percent higher. The oligomerization reaction willproceed over a wide range of conditions. Typical temperatures forcarrying out the reaction are between 0° C. and 430° C. Other conditionsinclude a space velocity from 0.1 to 3 LHSV and a pressure from 0 to2000 psig. Catalysts for the oligomerization reaction can be virtuallyany acidic material, such as, for example, zeolites, clays, resins, BF3complexes, HF, H₂SO₄, AlCl₃, ionic liquids (preferably ionic liquidscontaining a Bronsted or Lewis acidic component or a combination ofBronsted and Lewis acid components), transition metal-based catalysts(such as Cr/SiO₂), superacids, and the like. In addition, non-acidicoligomerization catalysts including certain organometallic or transitionmetal oligomerization catalysts may be used, such as, for example,zirconocenes. For illustration purposes reference is made to U.S. Pat.No. 6,703,535 which publication illustrates the preparation of a baseoil from a olefinic Fischer-Tropsch derived feed by means ofoligomerisation.

The invention is also directed to a process to prepare the intermediatefraction (ii). This fraction may be referred to as a waxy raffinatefraction boiling preferably for more than 90 wt % between 350 and 550°C., preferably between 370 and 550° C. The process to prepare thisintermediate product is preferably performed starting from aFischer-Tropsch synthesis product which boils for more than 40 wt % andmore preferably more than 50 wt % above 550° C. The process involves thefollowing steps:

(aa) separating the Fischer-Tropsch synthesis product into a fraction(i) boiling in the middle distillate range and below, a heavy endsfraction (iii) having an initial boiling point between 500 and 600° C.and a waxy raffinate fraction (ii) boiling between fraction (i) andheavy ends fraction (iii),

(bb) subjecting the heavy ends fraction (iii) to a conversion stepwherein part of the heavy ends fraction is converted to lower boilingcompounds and recycling the effluent of the conversion step to step(aa).

The prefered boiling range values for the fractions (i-iii) referred tohere above also apply for this process embodiment.

The conversion step (bb) may be the above referred to hydroconversionsteps as described for step (c) above. Alternatively thermal crackingmay also be applied to convert the heavy ends. Preferably the productsof the thermal cracker step (b) are hydrogenated such to at leasthydrogenate the di-olefins which could be present in said product.

The waxy raffinate may be sold as a separate product. For example it maybe prepared at a gas producing location and further processed to endproducts like for example base oils at a location more close to the endconsumers. The raffinate itself may find use as feedstock to preparebase oils as described above. The waxy raffinate product can alsoadvantageously be used as steam cracker feedstock to prepare lowerolefins, for example ethylene and propylene. Because of its highparaffin content, high yields to lower olefins are possible when usingsuch a feedstock as steam cracker feedstock.

Preferably the above process involving a thermal cracking step for step(bb) is used as a feed preparation process for a steam cracker locatedjust down stream of said process. A possible steam cracker process couldbe designed to run on a combination of said waxy raffinate product andon the lower boiling fractions (i), boiling in the ethane, LPG andnaphtha range and up to the gas oil range, as obtained in step (aa) indedicated steam cracker furnaces.

The invention will also be illustrated by making use of FIGS. 1-5

FIG. 1 illustrates a state of the art process of WO-A-02070629.

FIG. 2 illustrates a process according to the invention.

FIG. 3 illustrates a process according to the invention.

FIG. 4 illustrates a process according to the invention.

FIG. 5 illustrates a process according to the invention.

FIG. 1 describes a state of the art process line-up according toWO-A-02070629 illustrating a Fischer-Tropsch synthesis process step 1wherein a Fischer-Tropsch product 2 is prepared. This product 2 is fedto a hydrocracking/hydroisomerisation step 3. Product 4 is subsequentlyseparated in an atmospheric distillation column 5 into a naphtha product6, a kerosene product 7, a gas oil product 8 and a bottoms product. Thebottoms product is subsequently separated in a vacuum distillationcolumn 9 into a base oil precursor fraction 10 and a higher boilingfraction 17. The fraction 10 is subsequently catalytically dewaxed 11and the dewaxed oil 12 is fractionated in column 13 into various baseoil products 14, 15 and 16. The higher boiling fraction 17 is recycledto hydrocracking/hydroisomerisation step 3.

FIG. 2 illustrates an embodiment of the present invention. In aFischer-Tropsch synthesis process step 20 a Fischer-Tropsch product 21is prepared. This product 21 is separated by means of distillation 22 inone or more middle distillate fractions 34, 35, which may be naphtha,kerosene and gas oil, into a base oil precursor fraction 36 and a higherboiling fraction 23.

Distillation 22 may be a atmospheric distillation and a vacuumdistillation scheme as in FIG. 1. The base oil precursor fraction is fedto a catalytic dewaxing step 30 and the dewaxed oil 34 is fractionatedin column 32 into one or more base oil products 35, 36 and 37. Thehigher boiling fraction 23 is fed to a hydrocracking/hydroisomerisationstep 24 yielding a cracked product 25. From this product 25 a fractionboiling in the gas oil range and below 38, a base oil precursor fraction27 and a higher boiling fraction 33 is separated in column 26. The baseoil-precursor fraction 27 is catalytically dewaxed 28 and the dewaxedoil is combined with dewaxed oil 34 to be separated in 32 as describedabove.

FIG. 3 is a process as in FIG. 2 wherein the products obtained in thehydrocracking/hydroisomerisation step 44 is recycled to the firstseparation unit 42. As can be seen by comparing FIG. 2 with FIG. 3 aconsiderable reduction in unit operations is achieved. In aFischer-Tropsch synthesis process step 40 a Fischer-Tropsch product 41is prepared. This product 41 is separated by means of distillation 42 inone or more middle distillate fractions 46, 47, which may be naphtha,kerosene and gas oil, into a base oil precursor fraction 48 and a higherboiling fraction 43. Distillation 42 may be a atmospheric distillationand a vacuum distillation scheme as in FIG. 1. The higher boilingfraction 43 is fed to a hydrocracking/hydroisomerisation step 44yielding a cracked product 45, which is recycled to distillation 42.

The base oil precursor fraction 48 is fed to a catalytic dewaxing step49 and the dewaxed oil 50 is fractionated in column 51 into one or morebase oil products 53 and 54.

The gas oil product 52 as separated from the dewaxed oil is preferablyblended with the gas oil fraction 47 such to obtain a blended producthaving favorable low temperature properties. The gas oil product 52 willhave a low cloud point and cold filter plugging point (CFFP). The volumeof the gas oil product 52 having the favorable low temperatureproperties may be controlled by adjusting the initial boiling point ofthe base oil precursor fraction 48. Such a control allows the operatorto target the low volume of gas oil 52 and thus also the temperatureproperties, such as cloud point and CFFP of the resulting blend of gasoil products 52 and 47.

FIG. 4 illustrates a process to prepare a waxy raffinate product 65.FIG. 4 shows a Fischer-Tropsch synthesis process step 60 aFischer-Tropsch product 61 is prepared. This product 61 is separated bymeans of distillation 62 in one or more middle distillate fractions 63,64, which may be naphtha, kerosene and gas oil, into a waxy Raffinateproduct 65 and a higher boiling fraction 66. Distillation 62 may be anatmospheric distillation and a vacuum distillation scheme as in FIG. 1.The higher boiling fraction 66 is fed to a heavy ends conversion step 67yielding a product 68 containing on average lower boiling molecules thanthe feed 66. The heavy ends conversion step 67 may be any of the stepsdescribed above for step (c). The product 68 is recycled to distillation62. Optionally the waxy Raffinate 65 may be used as feed of a steamcracker furnace dedicated for such a feed. In a possible embodiment alsoa naphtha fraction 63 is fed to a dedicated steam cracker furnace. Thegas oil product 64 may be advantageously sold as a separate product.Because of its high Cetane Number it may be use, more advantageously asan automotive gas oil fuel component than as a steam cracker feed stock.

FIG. 5 is as FIG. 3 except that for step (c) a fluid catalytic crackingprocess (70) is performed. Fraction (64) is now rich in olefins andtherefore also gasoline fraction (63) which is used as feed to acatalytic oligomerisation-distillation column (71) is rich in olefins.The bottom stream (72) rich in compounds boiling in the base oil rangeis send to dewaxing step (49) to remove possible waxy compounds formedin column (71). The light compounds (74) are recycled to oligomerisationstep (71) and a bleed stream (73) is present to avoid a build up ofparaffins. Gas oil (47) may find application as ethylene crachesfeedstock industrial gas oil or as automotive gas oil.

The invention will be illustrated by the following non-limitingexamples.

EXAMPLE 1

A Fischer-Tropsch derived product having the properties as listed inTable 1 was distilled into fraction boiling substantially above 540° C.(recovered 72 wt % on feed to distillation) and a fraction boilingsubstantially between 350 and 540° C. (recovered as 25 wt % on feed todistillation). In addition 3 wt % of a fraction boiling substantiallybelow 350° C. was separated from the feed. The boiling curve data of thefeed and the main distillate fractions are listed in Table 1. TABLE 1Fischer- Tropsch derived 350° C.- product 540° C. 540° C. + (feed)fraction fraction (% weight fraction boiling below listed boiling point)Sample (% weight) 320° C. 5.5 5.8 1.6 350° C. 7.6 13.0 1.6 370° C. 9.322.2 1.6 400° C. 12.2 34.4 1.6 450° C. 17.4 64.3 1.6 500° C. 23.9 91.12.2 540° C. 29.5 99.0 6.6 590° C. 35.9 16.6 700° C. 51.6 43.6

The 540° C.+fraction of Table 1 was subjected to a hydrocracking stepwherein the feed was contacted with a 0.8 wt % platinum on amorphoussilica-alumina carrier. The conditions in the hydrocracking step were: afresh feed Weight Hourly Space Velocity (WHSV) of 0.9 kg/l.h, norecycle, and hydrogen gas rate=1100 Nl/kg feed, total pressure=32 bar.The reactor temperature was varied as listed in Table 2. Thehydrocracker effluent was analysed and the yields for the differentboiling fractions are listed in Table 2. TABLE 2 Example 1-a 1-b 1-c 1-dReactor 349   344   353   358   Temperature, ° C. Fraction 69.8 47.182.2 95.1 boiling below 370° C. Fraction 17.8 19.4 11.0 3.6 boilingbetween 370 and 540° C. Pour point +51   N.A. +57   N.A. of 350° C. plusfraction (° C.)N.A. = Not Analyzed

Thus relative to the feed to the distillation step 25 wt % of a fraction(i) boiling between 350 and 540° C. comprising substantially ofn-paraffins is obtained in the distillation step and 14 wt % of a waxyraffinate fraction (ii) boiling between 370 and 540° C. is obtained inthe hydrocracking step. These two fractions (i) and (ii) may be combinedand a base oil may be prepared from this combined fraction by dewaxing.

To calculate the potential base oil yield on these fractions (i) and(ii) we have used our conversion models which provide the followingconservative estimations. These estimations are to be used only in thecontext of the present application for illustration of the improved baseoil yield of the process of the present invention compared to a priorart process.

Base oil yield as the fraction boiling between 400 and 540° C. of thedewaxed oil, having a kinematic viscosity at 100° C. of 5 cSt and a pourpoint of −20° C. starting from a n-paraffinic feed (i) is 45 wt %. Thisyield is achievable when the base oils are obtained by a combinedhydroisomerisation step and a catalytic dewaxing step using a platinumZSM-23 catalyst as described in EP-A-776959 for a different feed.

Thus from fraction (i) 11 wt % on feed of base oil may be obtained.

Base oil yield as the fraction boiling between 400 and 540° C. of thedewaxed oil, having a kinematic viscosity at 100° C. of 5 cSt and a pourpoint of −20° C. starting from an waxy Raffinate feed (ii) is 70 wt %.This yield is achievable when the base oils are obtained by subjectingthe fraction (ii) to a catalytic dewaxing process using a silica boundplatinum ZSM-12 type of catalyst as described in U.S. Pat. No.6,576,120.

Thus from fraction (ii), as obtained in Example 1-b, 10 wt % on feed ofbase oil may be obtained. The total base oil yield on feed is thus 21 wt%.

Comparative Experiment A

Example 1 was repeated except that the Fischer-Tropsch derived product(feed) was directly submitted to the hydrocracker step. No priordistillation was performed. The yield to the 370-540° C. fraction onfeed was 24 wt %. Because this fraction is also partly hydroisomerisedthe same estimated base oil yield as for fraction (ii) of Example 1 maybe applied for this fraction. The base oil yield will then be 17 wt % onfeed.

As can be seen by comparing Example 1 and comparative experiment A isthat the base oil yield on Fischer-Tropsch derived product (feed) issignificantly higher for the process according to the present invention(=21 wt %) as compared to a situation wherein the prior art processline-up is used (=17 wt %).

1. A process to prepare base oils from a Fischer-Tropsch synthesisproduct, the processing comprising (a) separating the Fischer-Tropschsynthesis product into a fraction (i) boiling in the middle distillaterange and below, a heavy ends fraction (iii) and an intermediate baseoil precursor fraction (ii) boiling between fraction (i) and fraction(iii); (b) subjecting the base oil precursor fraction (ii) to acatalytic hydroisomerization and catalytic dewaxing process to yield oneor more base oil grades; (c) subjecting the heavy ends fraction (iii) toa conversion step to yield a fraction (iv) boiling below the heavy endsfraction (iii); and, (d) subjecting the high boiling fraction (v) offraction (iv) to a catalytic hydroisomerization and catalytic dewaxingprocess to yield one or more base oil grades.
 2. The process ofaccording to claim 1, wherein the heavy ends fraction (iii) has aninitial boiling point of between 500° C. and 600° C.
 3. The process ofclaim 1, wherein step (b) is performed in the presence of a catalystcomprising a noble metal hydrogenation component and a molecular sieveselected from the group consisting of zeolite beta, ZSM-23, ZSM-22,ZSM-35 or ZSM-12.
 4. The process of claim 1, wherein step (c) comprisesa hydrocracking/hydroisomerization process comprising contacting theheavy ends fraction (iii) with an amorphous catalyst comprising anacidic functionality and a hydrogenation/dehydrogenation functionality.5. The process of claim 1, wherein step (c) is performed under catalyticdewaxing conditions in the presence of a catalyst comprising a molecularsieve having a 12 member ring structure and a metal hydrogenationcomponents.
 6. The process of claim 5, wherein step (c) and (d) takeplace simultaneously.
 7. The process of claim 1, wherein step (d) isperformed in the presence of a catalyst comprising a noble metalhydrogenation component and a molecular sieve selected from the group ofzeolite beta, ZSM-23, ZSM-22, ZSM-35 or ZSM-12.
 8. The process of claim1, wherein the feed to step (a), step (b) and/or step (c) is firsthydrogenated.
 9. The process of claim 1, wherein step (c) comprises athermal cracking process.
 10. The process of claim 1, wherein step (c)comprises a catalytic cracking process.
 11. The process of claim 9,wherein the fraction boiling below 370° C. as obtained in step (c) issubjected to an oligomerization step (f).
 12. The process of claim 11,wherein a base oil fraction is prepared in step (f) and which base oilfraction is mixed with the base oil products obtained in step (b) and/or(d).
 13. The process of claim 11, wherein a base oil fraction isprepared in step (f) and which base oil fraction is dewaxed in step (b).14. The process of claim 1, wherein the effluent of step (c) is providedto step (a), such that in effect steps (b) and (d) take placesimultaneously.
 15. A process to prepare a waxy raffinate fractionboiling for more than 90 wt % between 370 and 550° C. from aFischer-Tropsch synthesis product which boils for more than 40 wt %above 550° C. by (aa) separating the Fischer-Tropsch synthesis productinto a fraction (i) boiling in the middle distillate range and below, aheavy ends fraction (iii) having an initial boiling point between 500and 600° C. and a waxy raffinate fraction (ii) boiling between fraction(i) and heavy ends fraction (iii), (bb) subjecting the heavy endsfraction (iii) to a conversion step wherein part of the heavy endsfraction is converted to lower boiling compounds and recycling theeffluent of the conversion step to step (aa).
 16. The process of claim2, wherein step (b) is performed in the presence of a catalystcomprising a noble metal hydrogenation component and a molecular sieveselected from the group consisting of zeolite beta, ZSM-23, ZSM-22,ZSM-35 or ZSM-12.
 17. The process of claim 2, wherein step (c) comprisesa hydrocracking/hydroisomerization process comprising contacting theheavy ends fraction (iii) with an amorphous catalyst comprising anacidic functionality and a hydrogenation/dehydrogenation functionality.18. The process of claim 2, step (c) is performed under catalyticdewaxing conditions in the presence of a catalyst comprising a molecularsieve having a 12 member ring structure and a metal hydrogenationcomponent.
 19. The process of claim 18, wherein step (c) and (d) takeplace simultaneously.
 20. The process of claim 2, wherein step (d) isperformed in the presence of a catalyst comprising a noble metalhydrogenation component and a molecular sieve selected from the group ofzeolite beta, ZSM-23, ZSM-22, ZSM-35 or ZSM-12.
 21. The process of claim2, wherein the feed to step (a), step (b) and/or step (c) is firsthydrogenated.
 22. The process of claim 2, wherein step (c) comprises athermal cracking process.
 23. The process of claim 2, wherein step (c)comprises a catalytic cracking process.
 24. The process of claim 23,wherein the fraction boiling below 370° C. as obtained in step (c) issubjected to an oligomerization step (f).
 25. The process of claim 24,wherein a base oil fraction is prepared in step (f) and which base oilfraction is mixed with the base oil products obtained in step (b) and/or(d).
 26. The process of claim 24, wherein a base oil fraction isprepared in step (f) and which base oil fraction is dewaxed in step (b).27. The process of claim 2, wherein the effluent of step (c) is providedto step (a), such that in effect steps (b) and (d) take placesimultaneously.
 28. The process of claim 10, wherein the fractionboiling below 370° C. as obtained in step (c) is subjected to anoligomerization step (f).